The invention relates to a method and apparatus for processing video pictures, in particular for large area flicker effect reduction.
More specifically the invention is closely related to a kind of video processing for improving the picture quality of pictures which are displayed on matrix displays like plasma display panels (PDP), display devices with digital micro mirror arrays (DMD) and all kind of displays based on the principle of duty cycle modulation (pulse width modulation) of light emission.
Although plasma display panels are known for many years, plasma displays are encountering a growing interest from TV manufacturers. Indeed, this technology now makes it possible to achieve flat colour panels of large size and with limited depths without any viewing angle constraints. The size of the displays may be much larger than the classical CRT picture tubes would have ever been allowed.
Referring to the latest generation of European TV sets, a lot of work has been made to improve its picture quality. Consequently, there is a strong demand, that a TV set built in a new technology like the plasma display technology has to provide a picture so good or better than the old standard TV technology.
A plasma display panel utilises a matrix array of discharge cells which could only be switched ON or OFF. Also unlike a CRT or LCD in which grey levels are expressed by analogue control of the light emission, in a PDP the grey level is controlled by modulating the number of light pulses per frame. This time-modulation will be integrated by the eye over a period corresponding to the eye time response. For static pictures, this time-modulation, repeats itself, with a base frequency equal to the frame frequency of the displayed video norm. As known from the CRT-technology, a light emission with base frequency of 50 Hz, introduces large area flicker, which can be eliminated by field repetition in 100 Hz CRT TV receivers.
Contrary to the CRTs, where the duty cycle of light emission is very short, the duty cycle of light emission in PDPs is xcx9c50% for middle grey. This reduces the amplitude of the 50 Hz frequency component in the spectrum, and thus large area flicker artefact, but due to the larger size of PDPs, with a larger viewing angle, even a reduced large area flicker becomes objectionable in terms of picture quality. The present trend of increasing size and brightness of PDPs, will also contribute to aggravate this problem in the future.
It is an object of the present invention to disclose a method and an apparatus which reduces the large area flicker artefact in PDPs in particular for 50 Hz video norms, without incurring extra costs similar to those required by 100 Hz TV receivers.
The reduction of the large area effect is made by utilising an optimised sub-field organisation for the frame period. The sub-fields of a pixel are organised in two consecutive groups, and to a value of a pixel a code word is assigned which distributes the active sub-field periods equally over the two sub-field groups.
This solution has the effect that the 50 Hz frequency component is substantially reduced compared to a sub-field organisation where only one sub-field group is used. The repetition of 50 Hz heavy lighting periods is substituted by a repetition of 100 Hz small lighting periods. By using this method virtually no extra costs are added, except for a slight increase in the PDP control complexity.
Advantageously, additional embodiments of the inventive method are disclosed herein. The use of identical structures for the two sub-field groups (for the most significant sub-fields) helps to make sure that the two lighting periods have similar characteristics. The weight of the least significant sub-fields is small and does not introduce significant large area flicker. This is the reason why it is not required that the least significant sub-fields are identical for the two sub-field groups.
In order to be able to display also non-standard video signals with variations in the horizontal line synchronisation signal, like the ones generated by video recorders or video games, a vertical blanking period has also to be used where no sub-field is addressed. Here, it is advantageous when this vertical blanking period is replaced by two vertical blanking periods, inserted between every pair of consecutive sub-field groups. This is similar to what happens in 100 Hz CRT based TV receivers.
The concrete sub-field organisation is advantageous for 50 Hz video norms. Compared to an optimised sub-field organisation for the 60 Hz video norms, like NTSC, there are more sub-fields used which is easily possible, because the frame period is longer.